The central theme of this program is to exploit recent advances in basic science for t he development of innovative gene therapy strategies, especially for metabolic disorders causing mental retardation. The program is highly focused on identifying and resolving the barriers to clinical gene therapy. In this period, five projects aim to exploit recent innovations t hat would enhance gene delivery and expression or actually correct genomic mutations in vivo. These projects are: Therapy for Hyperammonemia with Genetically-Engineered Bacteria (Tuchman). Adeno-Associated Virus Vector Treatment of Spinocerebellar Ataxia (McIvor) Chimeraplasty for Mutations Associated with Mental Retardation (Kren) Sleeping Beauty Transposon for Gene Therapy (Hackett) Lentiviral Ex Vivo Hematopoietic Stem Cell Gene Therapy for Mucopolysaccharidosis Type 1 (Whitley) The projects utilize a number of models of human metabolic disorders causing brain disease, notably, murine models of mucopolysaccharidosis, hyperammonemia, phenylketonuria and spinocerebellar ataxia. The program share core facilities for administration, microchemicals, quantitative PCR, hematopoietic cell processing, animal resources and viral vector production.